1. Field of the Invention
The present invention relates to a capacitive sensor that may be used to detect small traces of hydrocarbon molecules, and other selected analytes, in liquid medium. The uses of the invented sensor would include, but are not limited to: detecting trace hydrocarbon pollutants in ground water or streams; oil exploration; and, detecting trace substances in industrial and laboratory processes.
2. Description of the Prior Art
It has been the desire of engineers and scientists in the environmental, geological, and chemical process fields to have a reliable sensor to detect traces of hydrocarbon molecules in solutions. In the environmental field, such a sensor would detect hydrocarbon pollutants in ground water and streams, originating from industrial sites or storage tanks. In the geological field, such a sensor would be used to detect trace hydrocarbons in ground waters or at off shore sites to locate potential oil exploration areas. In the chemical process field, such a sensor would detect trace substances in industrial and laboratory processes. However, the prior art did not provide a sensitive and reliable sensor to detect such low concentrations of hydrocarbon analytes in solution.
Various prior art references teach detecting traces of water in pipes carrying oil. In U.S. Pat. No. 4,240,028 issued to Davis, Jr., and U.S. Pat. No. 4,517,547 issued to Gray et al, traces of water in oil are detected as the fuel mixture passes in a pipe through a capacitive sensor. The bulk dielectric of the oil in the pipe is increased by traces of water molecules. In U.S. Pat. No. 4,184,952 issued to Steward, a similar technique is used to measure basic sediment and water (BS&W) in crude oil. However, the above sensors do not have the sensitivity to detect low concentrations of hydrocarbons.
Various capacitive sensors are taught in the art to detect humidity in the air or anesthetic gas in a gas mixture delivered to a patient during various medical procedures. U.S. Pat. No. 3,350,941 issued to K. W. Misevich et al and U.S. Pat. Nos. 4,203,087 and 4,277,742 issued to Kovac et al, disclose capacitive humidity sensors. These humidity sensors use a moisture sensitive layer which absorbs water vapor. The dielectric property of the layer changes as it is penetrated by water vapors. As the moisture content of tne air increases the capacitance of the humidity sensor increases.
U.S Pat. No. 4,453,126 issued to G. A. Volgyesi and a published U.K. patent application GB No. 2 137 361 A, teach an apparatus for measuring anesthetic gas in a breathing mixture supplied to a patient. For both references, a planar capacitor is coated with a polymeric or lipid layer. The physical or chemical properties of the polymeric or lipd layer change with increasing concentration of the anesthetic gas, causing a change in the dielectric constant of the laver. These references report that an increase in the concentration of the anesthetic gas causes an increase in the capacitance of the sensor. The prior art anesthetic gas sensors use a reference sensor to compensate for the humidity of the gas mixture.